Microreactors, or continuous flow reactors having channels micrometer—up to tens of millimeter-scale minimum dimensions, offer many advantages over conventional batch reactors, including very significant improvements in energy efficiency, reaction condition control, safety, reliability, productivity, scalability, and portability. In such a microreactor, the chemical reactions take place continuously, generally in confinement within such channels.
To avoid safety problems in strongly exothermic reactions, it is proposed in the art to perform these reactions in a microreactor. In microreactors, these reactions are easier to control than in conventional batch reactors. In addition, it is possible in the microreactor to realize reaction conditions which are not realizable for safety reasons in a classical method in the laboratory or on the industrial scale. Robustness and chemical resistance of the micro-reactor body are therefore essentials when very corrosive compounds are employed (ie mineral acids or caustic solutions). Glass and ceramic materials (for example, Pyrex® glass, Aluminum oxide or Silicon Carbide), or glass-ceramics, are generally preferred for these conditions.
For glass, ceramic, or glass-ceramic microreactor, fluidic interconnection fittings are generally attached to microreactor fluidic module body through a seal compression system which employs a polymer o-ring tightened against an o-ring sealing location on the module body. The role of the o-ring is to ensure tightness but also to accommodate for stresses (such as small amounts of tube flexion or expansion mismatch on heating) which can lead to the failure of the refractory material of the fluidic module. Other techniques like brazing or welding can be applied, but these require very close matching of the coefficients of thermal expansion of the materials involved.
One type of seal in current use employs an o-ring against flat portion of the microreactor body, forming an o-ring face seal, such as shown and described for example, in FIG. 1 of EP1854543, a patent assigned to the present assignee. While the o-ring face seal shown in EP1854543 performs well enough under some conditions, there is a risk of the o-ring being extruded through the small remaining clearance between the o-ring seat and the fluidic module face. Another disadvantage is the need for external structures encircling the fluidic module or reaching around the edges thereof in order to compress the o-ring against the module face. Such structures can increase the complexity and cost, and the footprint of a reactor which typically includes a plurality of fluidic modules.
One alternative fitting system is known in the art as “o-ring boss seal”. FIG. 1 (prior art) shows a cross-section of an instance a fluidic interconnection system 10 employing an o-ring boss seal. A “boss” is cylindrical projection 22 on a structure 20, typically a cast or forged metal structure. The end 24 of the projection 22 is machined or otherwise formed to provide a flat, smooth surface 26 for sealing. Internal threads 28 are provided inside the bore of a fluidic port or opening 30 in the structure 20. A coupler 40 includes external threads 42 to mate with the internal threads 28, a flat annular sealing surface 44 to mate with smooth surface 26, and a gripping or engaging surface 46 for threading the coupler 40 into the structure 20. When the coupler 40 and the structure 20 are assembled together, an o-ring 50 is trapped in the annular space closed off by the contact of surface 26 and surface 44. Pressure from within the opening 30 merely wedges the o-ring more tightly between the coupler and a tapered inner surface 23 of the projection 22. A tube 60 is inserted within a compressible sleeve 48, which is compressed against the tube 60 by a compression nut 49 to retain the tube 60.
A fitting system 10 like that of FIG. 1 s preferable to the o-ring face seal system described above and in EP1854543 in at least two respects: (1) a pipe-thread style attachment is used, so no external fixtures are necessary for attachment of the fixture and compression of the o-ring 50, and (2) the o-ring 50, when in use, is seated into an enclosed cavity and cannot escape even at high pressure differentials. For at least these reasons, it would be desirable to employ with a refractory fluidic modules a fitting system similar to the fitting system 10 in FIG. 1, having an o-ring boss seal.
The main difficulty in employing a system similar to the fitting system 10 in refractory materials like glass, ceramic and glass-ceramic is that special tools and machinery are necessary to machine features without breaking the materials. In general, grinding machines with diamond-faced tools are required, and the glass or the ceramic part must be cooled with water to avoid excessive heat. In the case of very hard ceramics, such as SiC (silicon carbide), WC (tungsten carbide) or B4C (or B12C3) (boron carbide), such a process is very expensive because the process very slow and the special and expensive tooling is quickly worn down. As a consequence, the typical and preferred method to form these very hard materials is green-state machining—machining the ceramic in a green, unfired state. Unfortunately, the dimensional change on firing is too great for molding threads for fittings, since the shrinkage variation is about 1-3%. Green-state machining to rough tolerances followed by post-firing machining to final tolerance is possible, but still very expensive.
The present disclosure provides a method for forming complex structures in refractory body-based microfluidic modules, in particular, for forming screw threads within a ceramic body or within a portion or component of a ceramic body of a microfluidic device for mechanical fastening, particularly in order to secure a coupler for fluidic interconnection.
According to one embodiment of the disclosure, a method of forming complex structures in a ceramic-, glass- or glass-ceramic-body microfluidic module is provided, including the steps of providing at green-state refractory-material structure comprising least a portion of a body of a microfluidic module, providing a removeable insert formed of a carbon or of a carbonaceous material having an external surface comprising a negative surface of a desired surface to be formed in the microfluidic module, machining an opening in the green-state structure, positioning the insert in the opening, firing the green-state structure and the insert together, and after firing is complete, removing the insert. The insert is desirably a screw or screw shape, such that interior threads are formed thereby. The insert desirably comprises graphite, and the structure desirably comprises ceramic, desirably silicon carbide.
Certain variations and embodiments of the method of the present disclosure are described in the text below and with reference to the figures, described in brief immediately below.